<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>

<body>
<script>
    // 父类有加法方法
    class Father {
        constructor(x, y) {
            this.x = x;
            this.y = y;
        }

        sum() {
            console.log(this.x + this.y);
        }
    }

    // 子类继承父类加法方法 同时 扩展减法方法
    class Son extends Father {
        constructor(x, y) {
            // 利用super 调用父类的构造函数
            // super 必须在子类this之前调用
            super(x, y);
            this.x = x;
            this.y = y;

        }

        subtract() {
            console.log(this.x - this.y);

        }
    }

    const son = new Son(5, 3);
    son.subtract();
    son.sum();
</script>
</body>

</html>
